Auctiva automatically resizes to max (either dimension) of 1024 W x 768 H on upload, BUT also stores a downsized 400 x 300 for use in many of the templates. Note the 4:3 aspect ratio, which is common to most digital cameras. I guess it depends on exactly how you are displaying the images and the quality of your resize software.
My best advise is to upload original images to Auctiva, and let them resize and store appropriately. Try that as a test and see if it gives you better results. I usually use the 1.7MP setting on my cam for uncropped images, as it minimizes the upload time and gets just about the right amount of downsize without any pixilation by Auctiva. You can certainly use higher res, it just takes a little longer.
Danno